Manufacturing Practices (BT-106) - Important Questions

Define cold working and hot working of metals. Explain the effects of temperature on mechanical properties during deformation. List advantages and disadvantages of cold working and hot working with respect to ductility, strength, surface finish and dimensional accuracy.

Describe the forging process. Differentiate between open-die forging and closed-die (impression) forging. Explain press forging and drop forging with neat sketches. Mention common forging defects and their causes.

A circular rod of diameter $d_0=20\,\text{mm}$ is drawn to a final diameter $d_f=12\,\text{mm}$. Calculate:

1. The engineering (nominal) strain.

2. The true (logarithmic) strain.

3. Percentage reduction in area.

Use $A=\dfrac{\pi d^2}{4}$ wherever required.

Explain the extrusion process. Differentiate between direct (forward) extrusion and indirect (backward) extrusion. Discuss die design considerations and common extrusion defects with remedies.

A steel rod of diameter $25\,\text{mm}$ is drawn to a wire of diameter $10\,\text{mm}$ in a single pass. The average flow stress during drawing may be taken as $\sigma_f=350\,\text{MPa}$. Assuming negligible friction and elastic effects, calculate:

1. The true strain in drawing.

2. The ideal drawing force required ignoring friction.

Use $A=\dfrac{\pi d^2}{4}$ and express force in kN.

Describe common sheet metal forming operations: shearing, bending, drawing and stretching. For each operation, state the principle, tooling used, typical applications, and possible defects with causes and remedies.

Explain the deep drawing process. Describe stages of drawing and the factors affecting drawability. Outline the method to estimate the drawing force and state the significance of blank holder force.

Derive an expression for the ideal billet extrusion pressure based on homogeneous deformation and constant flow stress assumption using true strain concept. Using the derived relation, calculate the extrusion force for extruding a billet of initial cross-sectional area $A_0$ to area $A_f$ when average flow stress is $\sigma_f$. Express the final formula for force $F$ in terms of $\sigma_f$, $A_f$ and $A_0$.

List and explain common defects encountered in metal forming processes (forging, rolling, extrusion and drawing). For each defect give possible causes and suggest remedial measures.

Discuss the role of lubrication in metal forming processes. Describe types of lubricants used and criteria for lubricant selection in hot and cold working.

Explain the influence of strain rate and temperature on deformation behaviour during hot working. Define dynamic recrystallization and discuss its effect on grain structure and mechanical properties.

In a flat rolling operation a steel strip of thickness $h_0$ is reduced to $h_f$ in a single pass. Define the engineering reduction and true strain. If the mean flow stress during rolling is $\sigma_m$ and the width of strip is $w$, derive the expression for rolling force $F$ in terms of $\sigma_m$, $w$, $h_0$ and $h_f$. State assumptions made.